That's because sugars from fruit will stay on your fingers until you wash them with tap water, a new study shows.

Even alcohol swabs don't solve the problem.

The researchers say that peeling fruit right before you use a blood sugar meter, or eating some juicy fruit with your hands, could lead to an inaccurate reading even if you rub your finger with alcohol first.

Blood sugar meters work by taking a drop of blood from the tip of the finger and testing the sample for how much sugar is in the blood. Many people with diabetes have to use these meters a few times a day to monitor their blood sugar levels and determine how much insulin to take.

Close to 20 million people in the U.S. have diabetes, according to the Centers for Disease Control and Prevention.

For the new study, published in the journal Diabetes Care, Dr. Takahisa Hirose of the Juntendo University Graduate School of Medicine in Tokyo, Japan, and colleagues found ten healthy volunteers. None of the volunteers had diabetes; they all had normal blood sugar levels.

The researchers measured the blood sugar levels of all volunteers under a variety of conditions. First, participants swabbed a finger with alcohol and did the test before handling any fruit, to find their true blood sugar levels.

Next, the volunteers peeled oranges, grapes, or kiwis and checked their blood sugar without cleaning up, after cleaning up with alcohol, and after washing with tap water.

When fruit peeling was followed by hand washing, blood sugar readings were the same as before volunteers had peeled any fruit -- generally around 90 milligrams per deciliter (mg/dL), considered in the normal range.

But when volunteers peeled fruit and took a blood sugar reading right away, the levels shown by the blood sugar meter shot up -- to about 170 mg/dL on average after peeing an orange, 180 mg/dL after peeling a kiwi, and 360 mg/dL after peeling a grape.

And if the peelers swabbed their finger with alcohol in between peeling and finger pricking, the readings were still higher than normal. Even swabbing five times didn't produce correct results.

In the directions that come with blood sugar meters, the manufacturers generally tell users to wipe their finger with an alcohol swab before taking a reading. But that might not always do the job, the researchers say.

"People are used to pricking the finger, drawing a blood sample, (and) assuming that the measurement they make reflects the sugar content in the blood," said Dr. Robert Cohen, who studies diabetes at the University of Cincinnati Medical Center and was not involved in the current research.

But, he told Reuters Health, patients "really have to have a checklist" that helps them decide, Was the test done right? Are the circumstances right for this?

Cohen said the example in this study may seem obvious: having sugar on your fingers from fruit can interfere with that meter's readings. But it's still an important finding, he said.

When you use the meters, Cohen said, the assumption is that you're measuring your blood sugar, not the sugar on your fingers.

Getting an inaccurate reading from sugar on the outside of the finger "could lead somebody to take insulin when they don't need it," Cohen said. "They could run the risk of running a low sugar from having misleading data."

The message, the authors say, is to wash your hands before using a blood sugar meter and not rely on alcohol swabs, especially if you've been handling any fruit.